A hydrogen absorbing alloy is an alloy capable of safely and easily storing hydrogen as an energy source and therefore the alloy has drawn lots of attention as a new material for energy conversion and storage.
Application fields of the hydrogen absorbing alloy as a functional material have been proposed in a wide range such as storage and transportation of hydrogen, storage and transportation of heat, heat-mechanical energy conversion, separation and refining of hydrogen, separation of hydrogen isotopes, batteries using hydrogen as an active mass, catalysts for synthetic chemistry, and temperature sensors.
For instance, a nickel-hydrogen storage battery using a hydrogen absorbing alloy as a negative electrode material has following characteristics; (a) having a high capacity; (b) highly durable to overcharge and overdischarge; (c) capable of charging and discharging at high efficiency; and (d) is clean and therefore, the battery has drawn attention as a consumer battery and to further improve functions and capabilities (e.g. to improve charging and discharging cycle characteristics and capacities of batteries), its applications and practical uses have been actively promoted.
As an electrode material for a nickel-hydrogen storage battery, which is one application example of such a hydrogen absorbing alloy, are practically used AB5 type rare earth-Ni based alloys having a CaCu5 type crystal structure; however, the discharge capacity of the alloy is limited to about 300 mAh/g and it is difficult to further increase the capacity in the present state.
On the other hand, in recent years, various kinds of rare earth-Mg—Ni based alloys have drawn attention as new hydrogen absorbing alloys provided with durability which AB5 based hydrogen absorbing alloys have and a high capacity which AB2 based hydrogen absorbing alloys have in combination and it is reported that use of the alloys as an electrode makes it possible to have a discharge capacity exceeding that achieved by using an AB5 type alloy.
For instance, the following Patent Document 1 discloses electrodes containing LaCaMgNi9 alloys having a PuNi3 type crystal structure.
Patent Document 1: Japanese Patent No. 3015885
However, although having large hydrogen absorption capacities, the alloys described in Patent Document 1 have a problem that the alloys have low hydrogen releasing speeds (in other words, being inferior in the rate characteristics).
Further, Patent Document 2 discloses hydrogen absorbing alloys containing a phase of intermetallic compounds defined as La5Ni19 and additionally hydrogen absorbing alloys containing a phase of intermetallic compounds defined as (La-M)5Ni19(M:Ca, Mg). The hydrogen absorbing alloys described in Patent Document 2 are produced by mechanical alloying using two or more types of different hydrogen absorbing alloys as the material.
Patent Document 2: Japanese Patent No. 3397981
Further, the following Patent Documents 3 to 5 disclose that electrodes using rare earth-Mg—Ni based alloys having crystal structures such as a CeNi3 type, a Gd2Co7 type, a Ce2Ni7 type, and a PuNi3 type show good hydrogen releasing characteristics while keeping high hydrogen storage capacities.
Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 11-323469
Patent Document 4: JP-A No. 2002-273346
Patent Document 5: JP-A No. 2002-105563
Further, the following Patent Document 6 discloses that with respect to alloys having a Ce5Co19 type crystal structure, electrodes produced using the alloys compounded with rare earth-Ni alloys having a CaCu5 type crystal structure are excellent in terms of a hydrogenation reaction speed.
Patent Document 6: Japanese Patent No. 3490871
Further, other than the above-mentioned patent documents, as hydrogen absorbing alloys for providing hydrogen absorbing alloy electrodes with high capacities have been proposed many kinds of rare earth element-Mg—Ni based hydrogen absorbing alloys and hydrogen absorbing alloys containing the respective elements of rare earth element-Mg—Ni based alloys as main constituent elements and additionally elements such as Cu, Co, Mn, and Al as other constituent elements (e.g. Patent Documents 7 to 10).
Patent Document 7: JP-A No. 2000-80429
Patent Document 8: JP-A No. 2004-115870
Patent Document 9: JP-A No. 2000-265229
Patent Document 10: JP-A No. 2000-21439